(a) Field of the Invention
The present invention relates to a method and a system for cutting fuel for a hybrid vehicle.
(b) Description of the Related Art
A hybrid vehicle is defined as using two or more different power sources to efficiently drive a vehicle. Generally, hybrid vehicles include an engine generating torque by combusting fuel and a motor operated by a battery. Recently, hybrid vehicles have been developed and attracted attention in regards to their enhanced fuel economy and their reduced effects on the environment.
Hybrid vehicles often run in various modes (e.g., an electric vehicle mode, a hybrid mode, an engine mode, a regenerative braking (RB) mode, and so on) depending on a driving condition thereof. In the electric vehicle mode, the driving torque is generated only by the motor. In the hybrid mode, the driving torque is generated by the motor and the engine, and in the engine mode, the driving torque is generated only by the engine. In the regenerative braking mode, electrical energy is generated by the inertia from a vehicle and used to charge a battery for powering the motor. Accordingly, hybrid vehicles may use the mechanical energy of an engine, the electrical energy of a battery and may regenerate electrical energy from braking and thus may enhance fuel consumption.
FIG. 1 is a schematic diagram of a conventional transmission system (flexible hybrid system, FHS) of a hybrid vehicle. The system may realize two EVT (electrical variable transmission) modes and three FG (fixed gear) modes.
Referring to FIG. 1, the system includes an engine 10, a first planetary gear unit 20, a second planetary gear unit 30, a first motor/generator (MG1) 51, a second motor/generator (MG2) 52, a first clutch CL1, a second clutch CL2, a first brake BK1, a second brake BK2, and a transmission output shaft 80. The first planetary gear unit 20 includes a first sun gear S1, a first carrier C1, and a first ring gear R1. The second planetary gear unit 30 includes a second sun gear S2, a second carrier C2, and a second ring gear R2.
The engine 10 is connected to the first carrier C1, and the engine 10 rotates the first carrier C1 around the first sun gear S1. The first motor/generator 51 is disposed to rotate the first ring gear R1. The first brake BK1 selectively stops rotation of the first ring gear R1.
The first sun gear S1 and the second sun gear S2 are continuously connected to rotate together, and the second motor/generator 52 rotates the first and second sun gears S1 and S2.
The first clutch CL1 selectively connects the first carrier C1 with the first ring gear R1, and the second clutch CL2 selectively connects the first carrier C1 with the second ring gear R2. Furthermore, if both CL1 and CL2 are engaged, the first carrier C1 and the first R1 are connected to the second ring gear R2.
The second brake BK2 is disposed to selectively stop rotation of the second ring gear R2. The second carrier C2 is directly connected to the output shaft 80 of the transmission, and transfers output torque of the engine 10, the first motor/generator 51, and the second motor/generator 52 to the wheels.
In the transmission system, as shown in FIG. 1, when a vehicle starts or drives at a low speed, only the motor/generator 51 and 52 generate driving torque because generally engine efficiency is less than efficiency of the motor/generator 51 and 52 in a starting condition or in low speed driving. Thus, in a starting condition, fuel efficiency may be enhanced using the motor/generator 51 and 52 without operating the engine 10. After the vehicle begins moving, an ISG (Integrated start generator) starts the engine 10 and thus engine output and the motor/generator 51 and 52 output may be used simultaneously.
During braking, a hybrid vehicle may be operated in a regenerative braking mode as shown in FIG. 3. In this case, the second motor/generator 52 may generate inverse direction torque (charging torque) and thus electrical energy may be regenerated to the battery 70. That is, the second motor/generator 52 charges a battery 70 using the inertia of a vehicle. In this case, the charging torque is defined as the opposite direction torque of the second motor/generator 52 against a driving direction of a vehicle.
FIG. 2 is a graph illustrating the relationship between the vehicle speed of the vehicle and the torque when an accelerator pedal is not pushed (i.e., when a signal of an APS (accelerator-pedal position sensor) is “0”). In the drawing, the relationship of the vehicle speed and torque is shown for coasting driving.
When the vehicle is coasting (i.e., a driver is not pushing an accelerator pedal) in a general gasoline vehicle, a vehicle is operated by inertia of an engine and inertia torque (coasting torque) is applied to the engine. However, in the hybrid transmission system as shown in FIG. 1, for charging the battery 70 using driving torque of a vehicle in a coasting driving condition, the engine 10 is stopped and the second motor/generator 52 generates charging torque corresponding to the inertia torque which generated due to the inertia of the vehicle.
Accordingly, it is important to determine when to cut fuel to the engine when changing driving modes since fuel cutting time may influence drivability.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.